Oil fell on Thursday after U.S. President Donald Trump demanded OPEC cut crude prices, but the market found some support from an Iranian threat to block shipments through the Strait of Hormuz.
The escalating trade row between Washington and Beijing triggered another sell-off in Asian stocks and was felt in oil markets, with China warning it could introduce duties on U.S. crude imports.
Brent crude futures were at $77.89 per barrel at 0830 GMT, down 35 cents.
U.S. West Texas Intermediate (WTI) crude futures were down 11 cents at $74.03.
"If Trump continues to believe that OPEC are not doing enough, we would not rule out an SPR (Strategic Petroleum Reserve) release from the U.S., or possibly even export restrictions on petroleum products," ING said in a note.
"However with plenty of uncertainty over Iranian supply, and the Syncrude outage in Canada, the market is likely to remain fairly well supported in the near term."
Trump on Wednesday accused the Organization of the Petroleum Exporting Countries of driving up fuel prices.
"The OPEC Monopoly must remember that gas prices are up & they are doing little to help," Trump wrote on his personal Twitter account. "If anything, they are driving prices higher as the United States defends many of their members for very little $'s."
"This must be a two way street," he wrote, adding in block capitals, "REDUCE PRICING NOW!"
OPEC together with a group of non-OPEC producers led by Russia started to withhold output in 2017 to prop up the market.
Recent price rises have also been spurred by a U.S. announcement that it plans to reintroduce sanctions against Iran from November, targeting oil exports.
OPEC and Russia said in June they were willing to raise output to address concerns of supply shortages due to unplanned disruptions from Venezuela to Libya, and likely also to replace a potential fall in Iranian supplies due to U.S. sanctions.
Despite these measures, Goldman Sachs said in a July 4 note to clients that "the market will remain in deficit" in the second half of the year.
An Iranian Revolutionary Guards commander, meanwhile, said on Wednesday that Tehran might block oil shipments through the Strait of Hormuz, a major route for transporting crude in the Gulf.
"If they want to stop Iranian oil exports, we will not allow any oil shipment to pass through the Strait of Hormuz,” Ismail Kowsari was quoted as saying by the Young Journalists Club website. (Reuters)